The growing trend toward 3D imaging involving Volume Computed Tomography (CT) scanners and the use of 3D and Multi Planar Reconstruction (MPR) techniques leads to the need for phantoms and test methods that reveal to the radiologist and physicist actual 3D resolution, i.e., measures involving not only in-plane (x,y) resolution and related Modulation Transfer Functions (MTF's) but also slice width and Slice Sensitivity Profiles (SSP). These “combined” effects can be visualized and studied with the new resolution gauge of the present invention and are also amenable to analysis by automated approaches.
Resolution gauges have a long history of use in both photographic and radiological imaging. Repetitive patterns of contrasting bars and spaces (line pairs) at increasing frequency (repetition rates) are presented for visual (or computer) analysis for the observer to choose the pattern where no surviving contrast between the bars and spaces is perceived. In classical photography or TV images, the display image is basically a 2-D image (x,y plane) and therefore typical resolution gauges are also 2-D and, although usually arranged in a linear direction, can be arranged in other geometric presentations within the 2-D image space.
In CT, the images which are usually shown as 2D images, actually represent a set of given x,y (2D) information sets as results from averaging of all the 2-D planes within the z cut (slice width) of information. As such, there is a hidden or masked dimension (z) in the conventional 2D display of a 3D (x,y,z) acquisition of CT data. The Phantom Laboratory of Salem, N.Y. has produced CT resolution gauges that have a very thin (z-axis) extent, or maintained the periodic patterns of the resolution gauge constant in the z-axis direction, so that essentially the z-axis thickness had no effect on the perceived resolution or repetition rate. See for example, U.S. Pat. No. 5,164,978. Although this works well if one is interested in the resolution of an “infinitely thin” slice, it would not represent resolution aspects of the image when limited by the actual z-axis (slice thickness) cut.
In practice, most anatomical structures have variation in x,y, and z axis of the patient. Thus, current CT resolution gauges, in deliberately ignoring or minimizing the z-axis effect of the gauge, could tend to mislead the physician or observer to think that the resolution as seen from the gauge represents the resolution as might be achieved in the actual patient anatomy on a corresponding CT scan, rather than just in the 2D resolution gauge.
A typical axiom in imaging is that the smallest size object that might be seen from a given image of a resolution gauge is approximately, d˜1/(2 fc) where fc is the cutoff frequency, and d represents line spacing, thus a 0.5 mm object can usually be seen with a spatial frequency cutoff (fc) of 1 line pair/mm.
In the event that an object is “averaged” in a given slice thickness, the perceived contrast would be reduced and the smallest object that might be perceived would be considerably higher.